Unidirectional path switched ring
From Hill2dot0
A two-fiber configuration, the unidirectional path switched ring (UPSR) is a relatively simple SONET protection mechanism. In the absence of failure, a UPSR functions exactly like a basic single-fiber ring. All payloads are carried clockwise (CW) on a single fiber (link A). The payloads are duplicated counterclockwise (CCW) on a second fiber (link B), which is ignored in the absence of failure. At any one time, the receiver accepts payloads from only twelve STS-1s and any VTs inside the STS-1s.
Many local access applications still use UPSRs, and USPR is the only protection mechanism available for OC-3. However, a problem arises when UPSR is used for regional and national networks. Consider the CW link A on the visual. When ADM 1 sends to ADM 2, the signal arrives quickly across a single span. But when ADM 2 sends to ADM 1, the signal travels all the way around the ring. UPSRs have an asymmetrical delay (i.e., the outbound delay is not equal to the inbound delay), a situation that never occurs with physical point-to-point links, whether SONET or T-carrier.
If the ring is small enough and the application is voice, this delay is unimportant (which is not to imply that voice is delay-insensitive). However, data applications are different. Most data applications perform flow control through a windowing process. If the ring is large enough (e.g., some span several states and have up to 36 nodes on them), the difference between outbound and inbound delays can be substantial. For example, both ends could see a round-trip delay of 30 milliseconds, but for one it might be 5 milliseconds out and 25 milliseconds in, while the other experiences exactly the opposite. As one end of the data application adjusts its window size for a 15/15 millisecond split in delay, data continues to arrive 5 milliseconds early; as the other end adjusts the same way, data continues to arrive 25 milliseconds late.
The graphic uses CW and CCW for illustration only. In reality, ADMs are referenced by “transmit east” and “transmit west” (or similar terms).
UPSR vs. 2F-BLSR vs. 4F-BLSR
Although all fiber rings are capable of the protection switching that makes SONET rings attractive, there are some differences among them.
UPSRs and 2F-BLSRs are usually seen in cities, while 4F-BLSRs are seen in regional and larger installations.
UPSRs are the least complex and the least expensive to implement and expand. However, asymmetrical delay can be a concern for geographically large rings. If the STSs traverse many rings, the limitations of the “all or nothing” protection mechanism are exaggerated.
Both UPSRs and 2F-BLSRs protect against a single failure, but only 4F-BLSRs protect against multiple failures. Although 4F-BLSRs are more efficient, installation costs are higher. However, since 4F-BLSRs can be fully loaded, growth costs are lower. The ability to obtain revenue from “dark fibers” makes 4F-BLSRs attractive.
Compared with 4F-BLSRs and, especially, UPSRs, 2F-BLSRs are quite complex due to the spare channel that provides the protection in 2F-BLSRs. For this reason, 2F-BLSRs are losing popularity among carriers.
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